1. Field of the Invention
The present invention relates to a printed-wiring substrate and a method for fabricating the printed-wiring substrate. More particularly, the invention relates to a printed-wiring substrate including a dielectric resin layer, which forms a substrate surface, and to a method for fabricating the printed-wiring substrate.
2. Description of the Related Art
Conventionally, a printed-wiring substrate including a plurality of dielectric resin layers is known. Of the dielectric resin layers, a surface of an internal dielectric resin layer, which is formed in a substrate, is usually roughened at the surface thereof in order to enhance bonding strength for bonding to a conductor layer, such as a wiring layer, adjacent thereto, or to bond to another dielectric resin layer adjacent thereto. By roughening the surface of an internal dielectric resin layer, the enhancement of bonding strength for bonding to a conductor layer or the like can be expected from an anchoring effect provided by pits and projections formed on the roughened surface.
In contrast to an internal dielectric resin layer, the surface of an external dielectric resin layer (for example, a solder resist layer), which serves as the surface of a printed-wiring substrate, does not need to be roughened, since a conductive layer or another dielectric resin layer is not formed thereon. Accordingly, the substrate surface (surface of an external dielectric resin layer) has conventionally not been roughened.
3. Problems to be Solved by the Invention
Usually, a component number, a serial number, or the like of a printed-wiring substrate is marked on the surface of the printed-wiring substrate by means of a marking ink layer, such as an ink swatch or white paint. However, in some cases, because of low bonding strength between the substrate surface and the marking ink layer formed thereon, the marking ink layer tends to come off the substrate surface.
When an electronic component, such as an IC chip, is to be soldered onto the surface of a printed-wiring substrate, flux may be applied to substantially the entire substrate surface in order to remove oxides contained in solder to thereby enhance connection reliability. However, in some cases, the applied flux is repelled by the substrate surface and thus fails to uniformly spread over the entire substrate surface in a wetted condition.
When an IC chip or the like is mounted on a printed-wiring substrate, an under-fill may be filled (applied) into a gap between the IC chip and the printed-wiring substrate in order to enhance connection reliability therebetween. However, in some cases, the applied under-fill may fail to appropriately spread on the substrate surface in a wetted condition. This results in the formation of an unfilled portion of the gap between the IC chip and the printed-wiring substrate.
The present invention has been achieved in view of the foregoing. It is therefore an object of the invention to provide a printed-wiring substrate capable of suppressing a potential problem in processing a substrate surface, such as formation of a marking ink layer or application of flux or under-fill, as well as to provide a method for fabricating the printed-wiring substrate.
The above object of the present invention has been achieved by providing a printed-wiring substrate comprising an external dielectric resin layer which defines a substrate surface. The substrate surface is a roughened surface.
According to the present invention, the surface of the external dielectric resin layer, which serves as the surface of the printed-wiring substrate, is roughened.
Accordingly, when a marking ink layer, such as an ink swatch or white paint, is formed on the substrate surface, an anchoring effect provided by the roughened surface enhances bonding strength for bonding the marking ink layer and the substrate surface, thereby suppressing loss of the marking ink layer from the substrate surface.
When flux is applied to the substrate surface before an electronic component, such as an IC chip, is soldered to the substrate surface, flux is not repelled by the substrate surface, but spreads in a wetted condition, since the activity of the substrate surface is enhanced by roughening. Thus, flux can be uniformly applied to the substrate surface.
When an under-fill is filled into a gap between the substrate surface and an IC chip mounted on the substrate surface, the under-fill spreads properly in a wetted condition, since the activity of the substrate surface is enhanced. Thus, the under-fill reliably fills the gap.
A marking ink layer to be formed on the substrate surface may be a laminate composed of a plurality of marking ink layers, such as an ink swatch, or a single marking ink layer, such as white paint.
Preferably, the above-described printed-wiring substrate further comprises at least one internal dielectric resin layer, which is formed in a substrate. The substrate surface and the surface of the internal dielectric resin layer are roughened surfaces. The roughness of the substrate surface is lower than that of the surface of the internal dielectric resin layer.
As described above, the surface of the internal dielectric resin layer formed in the substrate is preferably roughened to a high level of roughness, in order to enhance bonding strength for bonding the internal dielectric resin layer and a conductor layer or for bonding the same and another dielectric resin layer.
However, if the substrate surface is roughened to a level of roughness as high as that of the internal dielectric resin layer, pits and projections of the roughened surface will become too deep and high for flux to be applied to the substrate surface or for an under-fill to be filled into a gap between a mounted IC chip and the substrate surface so as to spread in a wetted condition.
In contrast, according to the present invention, the substrate surface (the surface of the external dielectric resin layer) and the surface of the internal dielectric resin layer are roughened surfaces, but the roughness of the substrate surface is lower than that of the surface of the internal dielectric resin layer.
Accordingly, sufficient bonding strength can be established for bonding the internal dielectric resin layer and a conductor layer or for bonding the internal dielectric resin layer and another dielectric resin layer. Also, the marking ink layer can be formed on the substrate surface in a very strongly bonded condition, and flux or an under-fill can be reliably applied to the substrate surface.
Preferably, in the above-described printed-wiring substrate, a marking ink layer is formed on a portion of the substrate surface.
According to the present invention, the marking ink layer is formed on the substrate surface, which is a roughened surface. An anchoring effect provided by the roughened surface enhances bonding strength between the marking ink layer and the substrate surface. Thus, the marking ink layer is unlikely to come off the substrate surface, thereby enhancing reliability of the printed-wiring substrate.
Furthermore, when flux is applied to the substrate surface, the substrate surface can be reliably coated with the flux. Also, when an under-fill is filled into a gap between the substrate surface and an IC chip or the like, the gap can be reliably filled with the under-fill.
Preferably, in the above-described printed-wiring substrate, the marking ink layer comprises a first marking ink layer, which is formed on the substrate surface, and a second marking ink layer, which is formed on the first marking ink layer. The first marking ink layer absorbs laser radiation. The second marking ink layer assumes a color tone different from that of the first marking ink layer, and absorbs laser radiation to a higher degree than does the first marking ink layer.
In the printed-wiring substrate, the overlying second marking ink layer absorbs more laser radiation than does the underlying first marking ink layer. Thus, by irradiating the two-layered marking ink layer with, for example, a CO2 laser beam, a portion of the overlying second marking ink layer alone can be easily removed, thereby exposing the underlying first marking ink layer. Therefore, a marking step, such as marking a component number of the printed-wiring substrate, can be easily carried out.
The present invention further provides a method for fabricating a printed-wiring substrate comprising an external dielectric resin layer which defines a substrate surface, which method comprises roughening the substrate surface.
According to the present invention, the substrate surface (the surface of the external dielectric resin layer) is roughened.
Thus, when a marking ink layer is formed on a portion of the substrate surface, the marking ink layer can be formed in a very strongly bonded condition.
Also, when flux is applied to the substrate surface, flux can be reliably applied, since flux spreads in a good wetted condition. When an under-fill is filled into a gap between the substrate surface and an IC chip mounted on the substrate surface, the gap can be reliably filled with the under-fill, since the under-fill spreads in a good wetted condition.
The present invention still further provides a method for fabricating a printed-wiring substrate comprising an external dielectric resin layer which defines a substrate surface, and a connection pad exposed in an opening which extends through the external dielectric resin layer. The method comprises an external-dielectric-resin-layer roughening step for roughening the external dielectric resin layer among layers of the printed-wiring substrate, in which the connection pad is exposed in the opening of the external dielectric resin layer, which forms the substrate surface; a Nixe2x80x94Au-plating-layer forming step for forming, after the external-dielectric-resin-layer roughening step, an Ni plating layer on the connection pad exposed in the opening and then forming an Au plating layer on the Ni plating layer; and an ink-layer forming step for forming a marking ink layer on a portion of the substrate surface after the external-dielectric-resin-layer roughening step and before or after the Nixe2x80x94Au-plating-layer forming step.
According to the present invention, in the external-dielectric-resin-layer roughening step, the external dielectric resin layer (solder resist layer) among the layers of the printed-wiring substrate is roughened. Thus, the surface of the external dielectric resin layer (substrate surface) is roughened, and residue, such as resin, remaining in the opening formed in the external dielectric resin layer is removed. Therefore, in the subsequently performed Nixe2x80x94Au-plating-layer forming step, the Nixe2x80x94Au plating layer can be reliably formed on the connection pad exposed in the opening.
Furthermore, since the substrate surface is roughened before the ink-layer forming step, the marking ink layer can be formed on the substrate surface with a very high bonding strength in the ink-layer forming step.
Preferably, in the above-described method for fabricating a printed-wiring substrate, the ink-layer forming step comprises the steps of forming a first marking ink layer on the substrate surface; and forming a second marking ink layer on the first marking ink layer. The first marking ink layer exhibits low laser radiation absorption. The second marking ink layer assumes a color tone different from that of the first marking ink layer and absorbs laser radiation to a higher degree than does the first marking ink layer.
The thus-formed, two-layered marking ink layer allows easy formation of a marking, such as a component number of the printed-wiring substrate. Specifically, by irradiating the two-layered marking ink layer, for example, with a CO2 laser beam, a portion of the overlying second marking ink layer is removed to thereby expose the underlying first marking ink layer for forming a marking.
Preferably, the above-described method for fabricating a printed-wiring substrate further comprises a solder-bump forming step for forming a solder bump on the Nixe2x80x94Au plating layer on the connection pad exposed in the opening, after the Nixe2x80x94Au-plating-layer forming step and the ink-layer forming step, wherein the ink-layer forming step is an ink-layer printing step for forming a marking ink layer on a portion of the substrate surface by printing.
According to the present invention, the marking ink layer is formed by printing before the step of forming a solder bump.
Since solder bumps are not present on the substrate surface when the marking ink layer is formed, a printing mask can be reliably placed on the substrate surface. Thus, the marking ink layer can be reliably formed on the substrate surface by printing.